The present invention relates generally to electric motors, and particularly to elongated permanent magnet synchronous motors utilized in, for example, downhole, progressive cavity pumping systems.
In a variety of applications, it is advantageous to utilize an elongated motor having a relatively long stator and rotor mounted therein. For example, elongated induction motors are utilized in progressive cavity pumping applications, because they can be designed to fit within the confines of a wellbore. Induction motors are also beneficial in that they are not detrimentally affected by shaft twist due to loading of the elongated motor. However, the rotational speed of induction motors is relatively high and varies with load. In some applications, such as downhole progressive cavity (PC) pump systems, it would be advantageous to utilize a motor that operates at a lower rotational speed than an induction motor.
In PC Pump applications, for instance, conventional motors must be combined with a gearbox to reduce the rotational speed of the output shaft. A gearbox adds expense and complexity to the overall system.
Permanent magnet synchronous motors can be designed to operate at a lower speed that remains constant over certain ranges of variable load. However, an elongated permanent magnet synchronous motor is very susceptible to shaft twist. When a sufficient load is placed on the driving shaft, the resultant twisting tends to move the permanent magnets that are mounted on the rotors out of the optimal or desired rotational position relative to the stator. Permanent magnet synchronous motors are also difficult to start from a remote location. For example, if a permanent magnet synchronous motor is used in a downhole, wellbore environment, it is difficult to start the motor with a conventional controller disposed at the surface of the earth.
It would be advantageous to have a permanent magnet synchronous motor designed to self start in a downhole location and to compensate for the amount of shaft twist that occurs under normal loading.
The present invention features a permanent magnet synchronous motor system. The system comprises a permanent magnet synchronous motor including an elongated housing. A stator is disposed within the housing and includes a plurality of windings. Additionally, a rotor is rotatably disposed within the stator and includes a plurality of rotor sections. Each rotor section includes several permanent magnets that lie generally parallel with the axis of the rotor section. When the rotor sections are mounted on the shaft, the permanent magnets of adjacent or sequential rotor sections are offset from each other a predetermined angular displacement. The sum of the predetermined angular displacements is approximately equal to the angular displacement of the shaft under a given load.
According to another aspect of the present invention, a progressive cavity submersible pumping system is designed for use within a wellbore. The system includes a progressive cavity pump driven by a permanent magnet synchronous motor. Both the progressive cavity pump and the permanent magnet synchronous motor are designed for deployment in a wellbore to pump a wellbore fluid. The system also includes a variable speed drive able to output a three-phase alternating current. A power cable connects the variable speed drive to the permanent magnet synchronous motor to directly supply alternating current from a remote location to the submersed motor.
According to another aspect of the present invention, a method is provided for counteracting the effects due to shaft twist in an electric motor under load. The method includes mounting a first rotor section and a second rotor section within a stator. Additionally, the method includes attaching a first plurality of permanent magnets along the first rotor section and a second plurality of permanent magnets along the second rotor section. The first plurality of magnets is angularly offset with respect to the second plurality of magnets.
According to another aspect of the present invention, a progressive cavity pumping system is provided. The system includes a progressive cavity pump and a permanent magnet synchronous motor coupled to the progressive cavity pump. The output of the motor is directly coupled to the pump such that the pump rotates at the motor speed. In other words, the rotational speed of the permanent magnet synchronous motor and the progressive cavity pump are generally at a 1:1 ratio.